1. Field of the Invention
The present invention relates to the technical field of pre-pit detection apparatus for detecting pre-pits as pre-information, formed on a high-density recording medium such as a DVD-R (DVD-Recordable) or DVD-RW (DVD-Rerecordable), at the time of recording record information.
2. Description of the Related Art
In general, address information and reference signals for generating clock signals for use in recording and reproducing operations are pre-recorded on a recordable type optical recording medium in the form of pre-pits and pre-grooves in order to make it possible to record information on the medium. For example, on a DVD-R (Digital Versatile Disc-Recordable), pre-grooves as regions for recording information such as video and audio data intended to be recorded as recording pits are formed, while pre-pits (hereinafter called land pre-pits or LPP) are recorded on land portions each of which is formed between the pre-grooves.
In the operation of a pre-pit detecting device, reflected light of a light beam irradiated to a pre-groove is received at a light-receiving portion, which is divided into two parts by a division line at least optically parallel with a tangential direction of the pre-groove. As a result, first and second read signals are generated on the basis of the outputs of electrical signals from the light receiving portion, and the difference therebetween is computed to generate a difference signal (hereinafter called a xe2x80x9cradial push-pull signalxe2x80x9d). Then the radial push-pull signal is compared with a threshold by means of an LPP-signal binarizing circuit to obtain a signal extracted as a binary signal (hereinafter called an LPP signal).
In a conventional pre-pit detecting device, an AGC (Auto Gain Control) circuit makes the amplitudes of the first and second read signals coincide before generation of the above-mentioned radial push-pull signal. This configuration allows an RF component as a noise component to be effectively eliminated to increase LPP signal extraction performance even in extracting an LPP signal from the recorded DVD-R.
However, upon irradiating a high-power light beam for forming recording pits carrying information on a recordable type optical recording medium such as the above-mentioned DVD-R, thermal energy of the recording beam generally reduces the reflectivity at the irradiated position. In other words, the amount of reflected light at the position of a pre-groove on which a recording pit is formed (to which the recording beam has been irradiated) becomes smaller than the amount of reflected light at the position of a pre-groove on which no recording pit is formed.
On the other hand, upon recording information, the recording beam is irradiated to the overall or part of a land portion having an LPP existing therein and adjacent to the position of the pre-groove on which the recording pit carrying information is formed, which may form a recording pit there. In such a case, the reflectivity of the land portion in which the LPP exists is reduced, which may in turn reduce the amplitude level of the LPP signal component contained in the first and second read signals.
For example, FIG. 11 is an imaginary illustration showing the relationship between recording pit positions and waveforms of an LPP signal. FIG. 11A shows a case where a mark portion of the recording pit is overlapped on the overall LPP in the land portion. In this case, the amplitude level of the LPP signal component (solid-line portion) as a whole drops compared to the LPP signal component on which no recording pit is overlapped (broken-line portion).
FIGS. 11B and 11C show cases where the mark portion of the recording pit is overlapped on part of the LPP in the land portion. In these cases, the amplitude level of the LPP signal component (solid-line portion) drops only in the portion on which the recording pit is overlapped, compared to the LPP signal component (broken-line portion) on which no recording pit is overlapped.
On the other hand, since the above-mentioned AGC circuit uses average values of the above-mentioned read signal measured for a long time (about 10 ms) to perform AGC, it cannot correct the amplitude of a signal like the LPP signal component whose cycle is relatively short (about 100 to 500 ns). In other words, the conventional pre-pit detecting device has the above-mentioned AGC circuit insufficient to correct the amplitude of the LPP signal component whose amplitude level has dropped.
As a result, the conventional pre-pit detecting device cannot make clear the difference between the amplitude level of the LPP signal component and the amplitude level of a noise signal component remaining after generation of the radial push-pull signal, which causes a degradation in LPP signal extraction performance.
The present invention has been made in view of the above-mentioned problems, and it is an object thereof to provide a pre-pit detecting apparatus capable of correcting the amplitude of an LPP signal component contained in an RF component in a short time, and hence improving LPP signal extraction performance.
The above object of the present invention can be achieved by a pre-pit detecting apparatus of the present invention for an optical recording medium, which has information tracks with recording pits carrying record information formed thereon and guide tracks for guiding a light beam to the information tracks, and further has pre-pits carrying pre-information formed on the guide tracks. The pre-pit detecting apparatus including an amplitude correcting circuit, which when the optical beam is irradiated to one of the information tracks, corrects the amplitude of a first read signal and the amplitude of a second read signal generated on the basis of the outputs of electrical signals from a light receiving unit, divided into two parts by a division line at least optically parallel with the information track, such that the pre-pit detecting apparatus performs a computation on the difference between the corrected first and second read signals to generate a difference signal so as to detect a pre-pit signal on the basis of the difference signal. Further, said amplitude correcting circuit is provided with: an addition signal generating device for adding the first read signal to the second read signal to generate an addition signal; a first amplitude correcting device for correcting the amplitude of the first read signal, on the basis of the generated addition signal, for an RF signal component corresponding to a recording pit, which is contained in the first read signal; and a second amplitude correcting device for correcting the amplitude of the second read signal, on the basis of the generated addition signal, for an RF signal component corresponding to a recording pit, which is contained in the second read signal.
According to the present invention, the first and second read signals generated on the basis of the outputs of electrical signals from the light receiving portion are input to the amplitude correcting circuit in which the first and second read signals are added to generate the addition signal. The amplitudes of the first and second read signals are corrected, on the basis of the addition signal, for an RF signal component corresponding to a recording pit, which is contained in the first and second read signals. Then the difference between the first and second read signals corrected is computed to generate a difference signal so that a pre-pit signal will be detected on the basis of the difference signal.
Therefore, in the case of a recorded optical recording medium and even when the amplitude of the pre-pit signal component drops because of the decreased reflectivity of the pre-pit portion, the amplitude level can be corrected in a short time. Since the amplitude of the pre-pit signal can be corrected accurately, the difference between the amplitude level of the pre-pit signal component and the amplitude level of a noise signal component remaining after generation of the difference signal can be made clear, which makes it possible to improve pre-pit signal extraction performance. In addition, the circuit structure is simple, which can reduce manufacturing costs and the like.
In one aspect of the present invention, said first amplitude correcting device makes such a correction that the amplitude of the first read signal is momentarily amplified, on the basis of the generated addition signal, for an RF signal component corresponding to a recording pit, which is contained in the first read signal, and said second amplitude correcting device makes such a correction that the amplitude of the second read signal is momentarily amplified, on the basis of the generated addition signal, for an RF signal component corresponding to a recording pit, which is contained in the second read signal.
According to this aspect, in the case of a recorded optical recording medium and even when the amplitude of the pre-pit signal component drops because of the decreased reflectivity of the pre-pit portion, since the amplitude level can be momentarily amplified and corrected, the difference between the amplitude level of the pre-pit signal component and the amplitude level of a noise signal component remaining after generation of the difference signal can be made clear, which makes it possible to improve pre-pit signal extraction performance. In addition, the circuit structure is simple, which can reduce manufacturing costs and the like.
In another aspect of the present invention, said amplitude correcting circuit is further provided with an offset adjusting device that makes the neutral points of the first and second amplitude correcting devices coincide with each other.
According to this aspect, the pre-pit signal extraction performance can be improved more than the above mentioned invention.
In further aspect of the present invention, said amplitude correcting circuit is further provided with an addition-signal amplitude fluctuation-correcting device that corrects fluctuations in amplitude of the addition signal on the basis of whether the pit length of a recording pit carried with the addition signal is long or short.
According to this aspect, the fluctuations in amplitude of the addition signal can be corrected, which makes possible effective correction of the amplitude levels of the first and second read signals.
In further aspect of the present invention, said amplitude correcting circuit is further provided with: a first read signal delaying device that corrects phase shift between the addition signal and the first read signal, the phase shift produced as a result of correcting the fluctuations in amplitude of the addition signal by said addition-signal amplitude fluctuation-correcting device; and said second read signal delaying device that corrects phase shift between the addition signal and the second read signal, the phase shift produced as a result of correcting means correcting the fluctuations in amplitude of the addition signal by said addition-signal amplitude fluctuation-correcting device.
According to this aspect, even if phase shift is produced as a result of correction of the fluctuations in amplitude of the addition signal, the addition signal can be synchronized with the first read signal and the second read signal, respectively.
In further aspect of the present invention, aid amplitude correcting circuit is further provided with an addition-signal nonlinear amplification device that amplifies the addition signal to make the amplification factor for one polarity of the addition signal larger than the amplification factor for the other polarity.
According to this aspect, the signal-to-noise ratio of the pre-pit signal contained in the radial push-pull signal can be improved.